Famatinian orogeny

The Famatinian orogeny (Spanish: Orogenia de Famatina) is a process that created mountains and changed the land long ago. It happened in what is now western South America during a time called the Paleozoic. This event took place from the Late Cambrian to at least the Late Devonian, and maybe even a bit later, with the most activity happening about 490 to 460 million years ago.

During the Famatinian orogeny, the Earth's crust was changed by intense heat and pressure, called metamorphism and deformation. Magma, the hot liquid that comes from deep inside the Earth, erupted and pushed up into the crust along a line called a Famatinian magmatic arc. This created a chain of volcanoes.

The rocks formed from this magma are special. They are called igneous rocks and have a calc-alkaline character. Some of these rocks include gabbros, tonalites, granodiorites, and trondhjemites. The youngest rocks from this time are granites. Some of the pegmatite dykes in a place called the Pampean Pegmatite Province also formed during this time, and they might have come from special kinds of melted rock called S-type granitic melts.

Outcrops and sediments

The Famatinian orogen’s main visible parts are found in the Sierras Pampeanas in northwestern Argentina. Only the western part of this area shows signs of the Famatinian orogeny, while the eastern parts seem mostly unchanged. In northern Chile, the Belén Metamorphic Complex is believed to have experienced similar changes during the early Paleozoic, so it can be thought of as part of the orogen in a broader sense. In La Pampa Province to the south, there are few visible parts of the orogeny because newer layers of material cover most of the area.

In Peru’s Cordillera Oriental, there was a “Famatinian” orogeny that happened around the same time as the main one further south. Between 480 million and 435 million years ago, rocks in this area were bent and a chain of volcanoes formed.

To the east of the Famatinian volcanic chain, an old area of sediments turned into a special kind of basin during the Ordovician period. This basin stretched from Peru through Bolivia to northwestern Argentina. It collected sediments from the Famatinian orogen and volcanoes, and though it did not contain ocean floor, it was a sea basin.

Hard rocks that appear in Cordón de Lila and Sierra de Almeida south of Salar de Atacama in Chile formed during the Cambrian and Ordovician periods because of the orogeny. These rocks are types of granodiorite and monzogranite, and they are either metaluminous or peraluminous. They are leftovers from the volcanic activity along the western edge of the Famatinian orogeny.

Plate tectonic setting

The Famatinian arc was created when the Iapetus Ocean's lithosphere moved under Gondwana. As this process continued, the main event of the orogeny happened when the Cuyania terrane crashed into Pampia during the Ordovician period.

Some believe the Appalachian Taconic orogeny might be the northern part of the Famatinian orogeny. They think this is true because the continents Laurentia and Gondwana may have collided when the Iapetus Ocean closed. They also note that both areas have matching carbonate platform sediments and similar Olenellid trilobite faunas, which usually can't cross deep oceans. However, there are different ideas about where Cuyania came from and how it moved to its current place.